Typical of prior art rubber foams having heat resistance are silicone rubber foams which are undesirably poor in oil resistance. Fluorocarbon rubber, chloroprene rubber, and acrylonitrile-butadiene rubber (NBR) are known as foam rubbers having oil resistance. Among them, the foamed fluorocarbon rubber is less efficient to process and expensive, the foamed chloroprene rubber and NBR are low in heat resistance and weatherability because of the presence of numerous double bonds in their polymer molecule. There is a need for inexpensive rubber foams having high oil resistance, heat resistance, and weatherability as well as ease of molding.
Acryl rubber is a group of rubbery elastomers based on acrylate esters having excellent properties. Most often, these polymers have copolymerized therewith monomers having an active radical providing a crosslinking site. Based on the type of crosslinking radical providing monomer, the acryl rubbers are generally classified into a chlorine crosslinking type using chloroethyl vinyl ether, an active chlorine crosslinking type using vinyl chloroacetate, an epoxy crosslinking type using allyl glycidyl ether, and a non-chlorine crosslinking type using acrylic acid. When organic foaming agents which are decomposed at 40.degree. to 280.degree. C. to generate gas are used in these crosslinking types of acryl rubber, the balance between foaming and crosslinking/curing is lost, failing to produce a foam having a fine textured cell structure.
Only one successful acryl rubber foam is disclosed in Japanese Patent Application Kokai No. 82134/1980 and Japanese Patent Publication No. 15138/1984 as being obtained by copolymerizing ethyl acrylate with ethylidene norbornene in the presence of 2-mercaptobenzothiazole and vulcanizing the acryl rubber with sulfur in the presence of a pyrolytic foaming agent. Based on sulfur vulcanization, this foaming method requires addition of sulfur or a sulfur-containing vulcanizer, which inevitably gives off an offensive smell during foam preparation. In addition, sulfur vulcanized acryl rubbers suffer from losses of rubber elasticity, compression set, heat resistance, and durability.
Therefore, an object of the present invention is to eliminate the above-mentioned problems and to provide a novel and improved acryl rubber foaming composition which can be vulcanized with peroxide into an acryl rubber foam having a uniform fine independent cell structure without a need for sulfur or sulfur-containing vulcanizer. Another object of the present invention is to provide an acryl rubber foam having improved heat resistance, weatherability, crack resistance, and oil resistance.